1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical recording medium drive employing the same.
2. Description of the Prior Art
An optical pickup apparatus employed in an optical recording medium drive such as an optical disk drive records or reads information in or from an optical recording medium such as an optical disk or detects a servo signal with a laser beam.
Following recent requirement for miniaturization, weight reduction and cost reduction in relation to an optical pickup apparatus, research and development are made on an optical pickup apparatus employing a transmission-type holographic optical element, which is a kind of diffraction grating.
FIG. 25 schematically illustrates an optical pickup apparatus having a transmission-type holographic optical element, which is disclosed in Japanese Patent Laying-Open No. 3-76035 (1991). This optical pickup apparatus performs focus servo control with the astigmatic method and tracking servo control with the three-beam method.
Referring to FIG. 25, symbols X, Y and Z denote the radial direction of an optical disk 101, the track direction of the optical disk 101 and a direction perpendicular to the disk plane of the optical disk 101 respectively.
A semiconductor laser device 102 emits a laser beam (beam) in the direction Z. The beam emitted from the semiconductor laser device 102 is divided into a 0th order diffracted beam (main beam), a +1st order diffracted beam (subbeam) and a -1st order beam (subbeam) by a diffraction grating 103 in a plane substantially including the directions Y and Z, and transmitted through a transmission-type holographic optical element 104.
A objective lens 105 condenses the three beams transmitted through the transmission-type holographic optical element 104 on the optical disk 101 as a main spot M0 and subspots S1 and S2 positioned on both sides thereof. This objective lens 105 is supported to be movable in the direction X for a tracking operation as shown by arrow U, and movable in the direction Z for a focus operation.
The transmission-type holographic 104 diffracts the three returned beams (reflected beams) from the optical disk 101 in a plane substantially including the directions X and Z, so that a photodetector 106 detects these beams. The transmission-type holographic optical element 104 has a holographic surface 140 of an asymmetrical pattern as shown in FIG. 26, for supplying the three returned beams from the optical disk 101 with astigmatism respectively.
FIG. 27 illustrates the main spot M0 and the subspots S1 and S2 formed on the optical disk 101. As shown in FIG. 27, the optical system of the optical pickup apparatus is so adjusted that the main spot M0 scans a track TR to be reproduced and the subspots S1 and S2 scan parts located on both sides of the main spot M0 slightly over the track TR.
FIG. 28 is a typical plan view showing an exemplary structure of the photodetector 106. This photodetector 106 includes a four-segment photodetection part 160 provided on the central portion for performing focus servo control with the astigmatic method and photodetection parts E and F provided on both sides of the four-segment photodetection part 160 for performing tracking servo control with the three beam method. The returned beam (main beam) corresponding to the main spot M0 is incident on the central part of the four-segment photodetection part 160, while the returned beams (subbeams) corresponding to the subspots S1 and S2 are incident on the photodetection parts E and F respectively.
When the position of the optical disk 101 in the direction Z changes, the focal points of the returned beams change to change the shape of a light spot on the four-segment photodetection part 160 of the photodetector 106 as shown in FIG. 29. If the optical disk 101 is too close to the objective lens 105, the condensed spot S has an elliptic shape connecting the centers of photodetection parts B and D with each other along its major axis as shown in FIG. 29(a). If the optical disk 101 is on the focal (focused) position of the objective lens 105, the condensed spot S has a circular shape at the center of the photodetection part 160 as shown in FIG. 29(b). If the optical disk 101 is too far from the objective lens 105, the 10 condensed spot S has an elliptic shape connecting the centers of the photodetection parts A and C with each other along its major axis as shown in FIG. 29(c).
With output signals PA, PB, PC and PD from the photodetection parts A, B, C and D of the four-segment photodetection part 160, a focus error signal FES is obtained as follows: EQU FES=(PA+PC)-(PB+PD)
The above focus error signal FES becomes negative if the optical disk 101 is too close to the objective lens 105, becomes zero if the optical disk 101 is in an excellent focused state, and becomes positive if the optical disk 101 is too far from the objective lens 105.
When the main spot M0 excellently scans the track TR of the optical disk 101 to be reproduced, the two subbeams incident on the photodetection parts E and F are equal in intensity to each other. When the main spot M0 deviates to either side of the track TR to be reproduced, on the other hand, the intensity of either subbeam increases. With output signals PE and PF from the photodetection parts E and F, therefore, the following tracking error signal TES is obtained: EQU TES=PE-PF
FIG. 30 illustrates variation of the focus error signal FES with the position of the optical disk 101 in the direction Z. The variation of the focus error signal FES shown in FIG. 30 is called an S-curve characteristic. According to the astigmatism method, the amplitude of the S-curve characteristic can be increased with no loss in operation for obtaining an ideal S curve. Therefore, the astigmatism method is most widely employed in the optical pickup apparatuses which are put into practice at present.
The lasing wavelength of the semiconductor laser device 102 fluctuates depending on the environmental temperature. The diffraction angles of the returned beams on the transmission-type holographic optical element 104 change due to such fluctuation of the lasing wavelength.
In order to prevent fluctuation of the focus error signal FES resulting from the fluctuation of the lasing wavelength, the four-segment photodetection part 160 of the photodetector 106 is divided along a dividing line LX substantially along the moving direction of the diffracted beams on the holographic optical element 104.
Even if the photodetector 106 is arranged in consideration of wavelength fluctuation as hereinabove described, however, the condensed spot S of the diffracted beams deviates from the intersection between the dividing lines LX and LY following increase of wavelength fluctuation, and hence no correct focus error signal FES can be obtained in this case.
If the optical disk 101 is too close to the objective lens 105 and the lasing wavelength of the semiconductor laser device 102 increases, for example, the condensed spot S on the four-segment photodetection part 160 moves in the same direction (direction X) as the diffractive direction as shown by solid lines in FIG. 31. Consequently, the output signal PA from the photodetection part A increases to reduce the value of the focus error signal FES. Thus, the amplitude of the S-curve characteristic reduces to lower the detection accuracy for the focused state, as shown in FIG. 32.
In order to solve this problem, the inventors have employed a three-segment photodetection part 206 for focus servo control, which is divided by parallel dividing lines as shown in FIG. 33 in place of the aforementioned four-segment photodetection part 160 divided by the perpendicular dividing lines LX and LY, as a photodetection part for focus servo control.
With output signals S.sub.a, S.sub.b and S.sub.c from photodetection parts 206a, 206b and 206c, the following focus error signal FES is obtained: EQU FES=(S.sub.a +S.sub.c)-S.sub.b
In order to improve the S-curve characteristic indicating the amount of movement of the objective lens 105 in the optical axis direction and the intensity of the focus error signal FES, the apparatus must be so set that the condensed spot S of the diffracted beams uniformly spreads over the photodetection parts 206a and 206c. In case of employing the aforementioned photodetection part 206, however, it is difficult to adjust the apparatus so that the condensed spot S of the diffracted beams uniformly spreads over the photodetection parts 206a and 206c.
The inventor has further prepared an optical pickup apparatus shown in FIG. 34 and made experiments.
Referring to FIG. 34, a semiconductor laser device 302 emits a laser beam (beam) in a direction Z. A diffraction grating 303 divides the beam emitted from the semiconductor laser device 302 into three beams (a main beam and a pair of subbeams positioned on both sides thereof) in a plane substantially including directions Y and Z, and a transmission-type holographic optical element 304 transmits these beams. A objective lens 305 condenses the three beams transmitted through the transmission-type holographic optical element 304 on an optical disk 301 as a main spot and subspots positioned on both sides thereof.
The transmission-type holographic optical element 304 diffracts three returned beams (reflected beams) from the optical disk 301 in a plane substantially including the directions X and Z, so that a photodetector 306 detects these beams. The transmission-type holographic optical element 304 transmits the three beams, divides each of the three returned beams (reflected beams) from the optical disk 301 into two beams and diffracts the same, and supplies the divided beams with astigmatism corresponding to the focused state.
As shown in FIG. 35, the holographic optical element 304 has regions 304a and 340b divided by a dividing line 304L extending substantially along the track direction. FIG. 35 typically illustrates light spots m, s.sub.1 and s.sub.2 formed by the three beams M, S.sub.1 and S.sub.2 respectively.
As shown in FIG. 36, the photodetector 306 has a photodetection part 306a formed by photodetection parts 316a, 316b and 316c for detecting parts of the returned beam related to the main beam diffracted in the region 304a, a photodetector 306b formed by photodetection parts 316d, 316e and 316f for detecting parts of the returned beam related to the main beam diffracted in the region 304b, a photodetection part 306c for detecting parts of the returned beam related to the first subbeam diffracted in the regions 304a and 304b, and a photodetection part 306d for detecting parts of the returned beam related to the second subbeam diffracted in the regions 304a and 304b respectively.
On the basis of output signals S.sub.a, S.sub.b, S.sub.c, S.sub.d, S.sub.e and S.sub.f obtained from the photodetection parts 316a, 316b, 316c, 316d and 316f respectively and output signals S.sub.A and S.sub.B obtained from the photodetection parts 306c and 306d respectively, a focus error signal FES and a tracking error signal TES are obtained as follows: EQU FES=(S.sub.a +S.sub.c +S.sub.e)-(S.sub.b +S.sub.d +S.sub.f) EQU TES=S.sub.A -S.sub.B
Even if the condensed spot deviates from the centers of the photodetection parts 306a and 306b, therefore, an operation is made to correct this deviation to obtain a correct focus error signal FES.
When the objective lens 305 is moved in a direction (radial direction of the optical disk 301) substantially perpendicular to the track direction in the optical pickup apparatus employing the two-segment holographic optical element 304 as shown in FIG. 37 for moving the condensed spot on a desired track position, for example, the parts of the returned beams pass through portions of different sizes in the regions 304a and 304b of the holographic optical element 304 as shown in FIG. 38.
Therefore, the detected light quantities of the parts of the returned beam related to the main beam received in the photodetection parts 306a and 306b fluctuate following the radial movement of the objective lens 305. Thus, when the objective lens 305 radially moves by 0 .mu.m.+-.400 .mu.m, for example, S-curve characteristics are as shown in FIG. 39 and no correct focus error signal FES can be obtained.